Bob's ?? in Elkhard was selling that fiberglass by the foot in rolls we we were there in 09 so perhaps you could go to a local semi trailer repair shop to order it for you also.
good luck, watch your front axle weight in your design, those pickups are almost over empty.
2001 GMC 6500 Topkick, 22' box, dropped frame, designed to fit into a 9' garage door. 3126 CAT 6spd Man Lo-Pro 19.5's w/ 3.07 rear axle ratio